misha@nsc.UUCP (Michael Umansky) (10/28/86)
I am trying to get TRUE and COMPLETE information on function of
number of video heads in a VCR. The best theory I heard so far
is that one set of heads (2) has optimized head gap for Standard
Play (SP) mode and the second set of heads (2) has optimized head gap
for Extended/Long Play (EP/LP) and Super Extended/Long Play (SEP/SLP).
I would like to know if that is TRUE? If NOT, then how are the four (4)
or three (3) or five (5) heads used? If YES, then how do the three (3)
and five (5) head machines work? Please post to the net for general
audience information. Thanks in advance.
--
NAME: Michael Umansky (misha)
E-MAIL: ucbvax!hplabs!nsc!misha
WORK:
National Semiconductor Corporation
1135 Kern Avenue
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(408) 721-8109 (work)
HOME:
4331 Lincoln Way
San Francisco, CA 94122
(415) 564-3921 (home)perkins@bnrmtv.UUCP (Henry Perkins) (10/28/86)
> I am trying to get TRUE and COMPLETE information on function of > number of video heads in a VCR. > Michael Umansky (misha) Video magazine has an article about video heads roughly once a year. There are several different head designs, but here's the basic story for VHS. The minimum number of heads necessary for a VCR is two. Each head does one half frame, and the heads are 180 degrees apart on the head cylinder. However, a given head width can only be optimized for one speed of play. Cheap machines will perform well at only one speed (often EP), with adjacent tracks bleeding at a slower speed, and inter-track noise showing up at a higher speed. That's why all the machines with decent performance at more than one speed have a second pair of heads. Then you have one pair for SP and one pair for EP. Frequently a single extra head is added to better support special effects like freeze frame and slow motion. This gives 3 heads on the cheap machines, and 5 on the better ones. Things get more complicated from there. In some machines LP is done by combining one SP head with one EP head. Many otherwise good machines don't even allow recording at LP speed. As far as I know no manufacturer has three separate pairs of video heads to support the three recording speeds. For HiFi, where the two audio tracks are laid down "under" the video tracks, add two more heads. That's how you can get 6 or 7 heads. More properly, these are 4+2 or 5+2 heads, since you don't get any better picture by adding the HiFi heads. (Normal linear audio is recorded on the edge of the tape by a single non-moving head.) Beta is much simpler. You still have two opposed heads to record the picture. However, the tolerances are not as restrictive, so you can handle both Beta II and III speeds with the same pair of heads. Beta HiFi records the two audio tracks with the video signal, not "under" it. You need more circuitry, but you use the same two heads. Except for the very-top-of-the-line-with-all-the-editing-effects models, SuperBeta HiFi machines still only have two heads. The fanciest machines (like my Sony SL-HF900 SuperBeta) have 4 heads. A two-head SuperBeta VCR (costing about $350) has 20% more resolution than a 5+2 head $1400 VHS HQ machine. It's a shame Sony can't market their products as well as JVC. -- {hplabs,amdahl,3comvax}!bnrmtv!perkins --Henry Perkins It is better never to have been born. But who among us has such luck? One in a million, perhaps.
ugfailau@sunybcs.UUCP (Fai Lau) (10/28/86)
In article <3855@nsc.UUCP>, misha@nsc.UUCP (Michael Umansky) writes: > I am trying to get TRUE and COMPLETE information on function of > number of video heads in a VCR. The best theory I heard so far > is that one set of heads (2) has optimized head gap for Standard > Play (SP) mode and the second set of heads (2) has optimized head gap > for Extended/Long Play (EP/LP) and Super Extended/Long Play (SEP/SLP). > I would like to know if that is TRUE? If NOT, then how are the four (4) > or three (3) or five (5) heads used? If YES, then how do the three (3) > and five (5) head machines work? Please post to the net for general > audience information. Thanks in advance. > -- That is TRUE. The only reason VCRs have more than one set of heads is because the additional set has different size head gaps. In EP/LP mode, the video tape is travelling at a slower speed, and the tracks written by a standard set of video heads tend to overlap. (I assume you know the mechanics of video recording) So a set of heads with a smaller head gaps work better. Also, in slow mode with SP, the EP/LP heads are often used instead to read the tracks created by the SP heads. It is because when the tape speed slows down, the offset angle of the tracks originally written in regular speed can no longer agree with the angle of the video drum. And a head with smaller gap (OOPS! not just different gap size, but different THICKNESS too) and smaller thickness could minimize the distortions created by a SP head crossing over two adjacent tracks during scanning. That is, the smaller heads can fit into the path of a wider track even when it does not travel at exactly the same angle. I can not speak for ALL machines with odd number video heads, however, I can speak for some of them. In a three head machine, the additional head works WITH the standard pair of heads. It is only used during slow mode (or fast mode, as long as the speed is different). This head is usually mounted close to either one of the standard heads and is identical (maybe thinner) to it EXCEPT that its gap angle (video heads have tilted gaps, unlike audio eads) is aimed at the opposite direction, just like the other video head in the opposite side (should I go into details explaining why, I guess not, it would make the article too long). In these machines, every other track is skipped during fast mode. Since all tracks scanned this way were written with only one video head and are now scanned by two, the two heads used for scanning should have same gap angles. The mechinics of the five head machine is roughly the same. Fai Lau ECE / CS SUNY at Buffalo (The Arctic Wonderland) BI: ugfailau@sunybcs that the additional
feb@cblpe.UUCP (Franco Barber) (10/30/86)
In article <819@bnrmtv.UUCP> perkins@bnrmtv.UUCP (Henry Perkins) writes: >[... explanation of function of heads in VHS machines] >Frequently a single extra head is added to better support special >effects like freeze frame and slow motion. This gives 3 heads on the >cheap machines, and 5 on the better ones. >[... a little more stuff] >Except for the very-top-of-the-line-with-all-the-editing-effects >models, SuperBeta HiFi machines still only have two heads. The >fanciest machines (like my Sony SL-HF900 SuperBeta) have 4 heads. > > >A two-head SuperBeta VCR (costing about $350) has 20% more resolution >than a 5+2 head $1400 VHS HQ machine. It's a shame Sony can't market >their products as well as JVC. >-- >{hplabs,amdahl,3comvax}!bnrmtv!perkins --Henry Perkins > >It is better never to have been born. But who among us has such luck? >One in a million, perhaps. I thought I would take a minute to point out how the 900 uses its 4 video heads (I have one, too.) Two heads are used for BI, two heads are used for BII/BIII, optimized for BII (I've noticed picture search at BII is perfectly clear, but at BIII there are small snow bars in the picture.) (I haven't been able to test BI: I don't have any BI tapes to play. Maybe next month when I get my SL-HF1000 which records at BI I'll find out what BI is like on the 900.) Apparently, special effects and freeze frame on the 900 is done with only one head. There is a control on the top labeled "still adjust." With the machine in freeze frame, turning this control makes the two fields in the picture on the screen move in relation to each other; the scan lines in one field move up and down while the other field stays still. The only explanation I have for this is that perhaps one head (whichever head happens to line up with the current field) is used to read one field, which is displayed and stored in a CCD array (the kind of analog storage device also used in some video cameras) to be redisplayed on the next field after a time delay which is controlled (partially) by the "still adjust" control. This is of course, only an educated guess, based on the effects of the "still adjust" control and an old "VIDEO" magazine review which said two heads were used for BI and two for BII/III. This same 'storage' effect could be used on VHS, I suppose (and indeed, the just introduced 'digital' (:-) VHS vcr's do just that, except that they digitize the field into a frame buffer instead of using an analog delay line. ) -- Franco Barber ..!cbosgd!cbuxc!cblpe!feb